Sending unit terminal nut size????
Run it till it reads 1/4 then fill it up. Record how many gallons you get in it.
Basically they tend to read full for the top 5 gallons, then 1/2 for the next 5 gallons the 1/4 to empty for the last 10 gallons. (Give or take)
If you want to pass on your old sender PM me.